Azeotropic composition

ABSTRACT

THIS INVENTION RELATES TO AZEOTROPIC MIXTURES OF TETRACHLORODIFLUOROETHANE AND METHYL ETHYL KETONE.

United States Patent US. Cl. 252-171 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to azeotropic mixtures oftetrachlorodifluoroethane and methyl ethyl ketone.

This is a division of application Ser. No. 772,046, filed Oct. 30, 1968which in turn is a continuation-in-part of application Ser. No. 677,738,filed Oct. 24, 1967 which in turn is a continuation-in-part ofapplication Ser. No. 590,227, filed Oct. 28, 1966.

This invention relates to an azeotropic composition and particularly tothe azeotropic mixture of tetrachlorodifluoroethane and either one ofthe compounds ethanol, isopropanol, n-propanol, methyl ethyl ketone ormethyl n-propyl ketone.

Several of the chlorofiuoroetha-nes have attained widespread use asspecialty solvents in recent years, particularlytetrachlorodifluoroethane. This is a relatively high melting compound(CCl -FCCl F, 24.5 C.) which is nontoxic and nonflammable, and which hassatisfactory solvent power for greases, oils, waxes and the like undercertain conditions. It has therefore found widespread use in cleaningelectric motors, compressors, oxygen storage tanks, photographic film,lithographic plates, typewriters, instruments, gauges, sound tape, andas non-corrosive brines.

For certain solvent purposes however, the chlorofiuoroethanes alone haveinsufficient solvent power. This is particularly true in the electronicindustry during the manufacture of printed circuits. Printed circuitsare well known in the electronics art; and consist of a circuit formedfrom a soft metal on a solid, non-conducting surface such as areinforced phenolic resin. During manu facture, the solid surface iscoated with the metal, the desired portion of metal is coated with animpervious coating, and the excess metal is removed by etching with asuitable acid. After the excess metal has been removed, it is necessaryto remove the impervious coating because solder joints must be made tothe printed circui and these will not form if the coating is present.After the impervious coating is removed, the circuits are coated with arosin flux to permit the joints to be soldered, after which the rosinflux must then be removed.

The chlorofluoroethane solvent does not have suflicient solvent power toclean printed circuits; that is, to effectively remove the rosin flux.Although mixtures of solvents may be used for this purpose they have thedisadvantage that they boil over a range of temperatures andconsequently undergo fractionation in vapor degreasing or ultrasonicapplications which are open to the atmosphere. When employing either ofthese methods the solvent must also be substantially nontoxic andnonflammable for safety reasons.

Tetrachlorodifiuoroethane is a relatively high boiling fluorocarbon andfor this reason especially advantageous in vapor degreasing applicationssince at these temperatures the hot vapor has more of a tendency todissolve 3,737,390 Patented June 5, 1973 high melting greases, or fluxesas well as oil residues and the like. When articles such as circuitboards are passed through a vapor degreaser, the solvent vapors tend tocondense on the article until the articles are heated by the vapors fromroom temperature up to the temperture of the vapor. The condensationthus formed on the articles tends to drip back into the solventreservoir taking with it some of the soil on the article. For thisreason the ability of a cleaning solvent to condense on the surface isespecially advantageous. Higher boiling solvents prolong thiscondensation effect in a continuous degreaser since it takes a greateramount of time to bring the article passing through the degreaser up tothe vapor temperature of the solvent. Consequently higher boilingsolvents generally have better cleaning power per unit of time in acontinuous vapor degreaser than the lower boiling solvents.

Tetrachlorodifluoroethane also is a better solvent than materials suchas trichlorotrifiuoromethane however, it suffers the disadvantage thatit is solidus at room temperature Whereas the latter is liquidus.Accordingly tetrachlorodifluoroethane is more difficult to handle thanliquid type cleaning solvents.

It is an object of this invention to provide a constant boiling orazeotropic solvent that is a liquid at room temperature, will notfractionate and also has the foregoing advantages. Another object is toprovide an azeotropic composition which is valuable as a solvent andparticularly for cleaning printed circuits. A further object is toprovide an azeotropic composition which is both relatively nontoxic andnonflammable both in the liquid phase and in the vapor phase and whichat the same time is an excellent solvent for cleaning printed circuitsespecially by means of a continuous vapor degreasing machine.

The above object of this invention may be accomplished by a novelazeotropic composition of tetrachlorodifluoroethane (e.g. l,1,2,2tetrachloro 1,2 difluoroethane) and either one of the compounds ethanol,isopropanol, n-propanol, methyl ethyl ketone or methyl-npropyl ketone.Although 1,1,2,2 tetrachloro 1,2-difluoroethane is a preferredtetrachlorodifluoroethane, the isomer 1,l,l,2 tetrachloro 2,2difluoroethane may be substituted therefore in whole or in part andespecially in minor amounts or trace amounts. All of these mixtures formazeotropes which distill at a constant temperature, the liquid phase andthe vapor phase in equilibrium therewith having the same composition.Such mixture is relatively nonflammable and nontoxic in both the liquidphase and the vapor phase. These mixtures are particularly useful assolvents for greases, oils, waxes, and the like and cleaning electricmotors, compressors, photographic films, lithographic plate,typewriters, precision instruments, gauges, sound tapes, and the likeand are particularly useful for cleaning printed circuits.

Although the azeotropic mixtures are obtained at approximately 760 mm.Hg of a variation in pressure and consequently a change in thecompositions and boiling points are also intended to be Within the broadscope of the invention. Thus the azeotropes may contain many differentproportions of all of the aforementioned components provided a constantboiling mixture is obtained at the various pressures at which thecompositions are used. Stated otherwise any pressure may be employed toobtain the azeotropes of this invention as long as a two componentconstant boiling mixture is obtained, and accordingly the ratio ofcomponents of the azeotropes of the invention will also vary. Thevariation of components is thus Within the skill of the art and iseasily determined once it is known that the halogenated hydrocarbons ofthis invention will form the aforementioned azeotropes. In a preferredembodiment the present invention relates to the aforementionedazeotropes that boil at atmospheric pressure i about 25, especiallyabout mm. Hg.

EXAMPLE I Compo- Boiling sition, point,

percent C.-760

Example Component by weight mm. Hg

1 {1,1,2,Z-tetraehloro-l,2-difluoroethane- 75. 6 Ethanol 24.4 71

EXAMPLES II-V The method of Example I is repeated using differentmixtures of alcohols and ketones, the results of which are as follows:

nol-formaldehyde resin or an epoxy resin. Printed circuits are preparedby a variety of methods. In a typical procedure, the board consistsoriginally of a phenolic resin impregnated base to which is bonded asheet of copper, 2 to 4 mils thick, covering one surface of the board.The desired circuit is drawn on the copper with an asphalt based inkusing the silk screen method. The excess copper is then removed byetching with a ferric chloride-hydrochloric acid bath, sometimescontaining ammonium chloride, leaving on the board the copper that iscovered by the ink. After washing off the etch solution, the asphalt inkis removed by cleaning with the azeotropic composition of this inventionin an ultrasonic bath (some mechanical scrubbing is often used). Theentire surface of the board is coated with a rosin flux and dried. Theelectronic components (resistors, capacitors, etc.) are then added atthe proper places for soldering to the circuit. The board is then passedover a molten solder bath, contacting the desired joints with the moltenmetal, whereby the soldering is effected. After cooling, the excessrosin flux remaining on the board must be removed since, if present inthe final assembly, it will lead to corrosion, poor electricalresistance and other deleterious properties.

The board is cleaned by placing it in an ultrasonic bath of any of theaforementioned azeotropes and operating at Components, percent by weightBecause the aforementioned azeotropes exist at pressures other thanambient pressures, as mentioned previously, the components of themixtures may vary as follows:

Components parts by weight 1,1,2,2-tetraehloro-l,2-difluoro ethane--.75.6 =1: about 10% especially :1: about 5%.

Ethanol 24.4 :1: about 10% epsecially :1: about 5%.

1,1,2,2-tetrachloro-l,2-difluoro ethane 86.5 :1; about 10% especially:1: about 5%.

n-Propanol 13.5 :1: about 10% especially =l=about 5%.

1,1,2,2-tetrach1oro-1,2-difiuoro ethane 72.2 :1: about 10% especially:1: about 5%.

i-Propanol 27.8 about 10% especially :1: about 5%.

1,1,2,2tetraehloro-L2-difiuoro ethane..- 14.0 :1: about 10% especially5: about 5%.

Methyl ethyl ketone 86.0 =1; about 10% especially :1: about 5%.

1,1,2,2-tetrachloro-1,2-difluoro ethane... 93x 5: about 10% especially:1: about 5%.

Methyl n-propyl ketone 7 :1: about 10% especially :1; about 6%.

Thus where the azeotrope contains 75.6 parts by weight of1,1,2,2-tetrachloro-1,Z-difluoroethane i about 10% as noted above, it isintended that this component may vary from about 68.1 to about 83.1parts by weight and so forth for the rest of the ranges of all the otherabove components and their equivalents.

Printed circuit boards are usually prepared by impregnating glass cloth,nylon, or paper laminates with a pheabout 32 kilocycles per second atabout 38 C., where it remains for about one minute.

When the board is cleaned with the azeotropic mixtures of this inventionsubstantially all of the rosin flux is removed without any detrimentaleffect on the board which constitutes the backing of the printedcircuit.

Although the invention has been described by reference to some preferredembodiments it is not intended that the broad scope of the novelazeotropic compositions be limited thereby but that certainmodifications are intended to be included within the spirit and broadscope of the following claims.

What is claimed is:

1. An azeotropic mixture consisting essentially of about 14 parts of1,1,2,2-tetrachloro 1,2 difluoroethane and about 86 parts of methylethyl ketone on a weight basis, having a boiling point of 79 Centigradeat approximately 760 mm. Hg.

2. A method for cleaning a solid surface comprising contacting saidsurface with the composition of claim 1.

References Cited UNITED STATES PATENTS 2,999,815 9/1961 Eiseman 252DIG 93,085,116 4/ 1963 Kvalnes 252DIG 9 3,499,047 4/ 1970 Cormany et a1. 252DIG 9 WILLIAM E. SCHULZ, Primary Examiner U.S. Cl. X.R.

25266, DIG 9; 260-652.5

